Circulation in a Jet Being Cooled at the Surface

Abstract

The effect of cooling on an eastward-flowing jet is explored using simple quasigeosrophic (QG) theory. The effects are quantified in terms of a cooling-induced turning with depth, similar to that of Schott and Stommel. The turning with depth is explained as changes in the stretching terms of the QG equations. A two-layer QG model of an eastward-flowing jet is formulated and solved numerically. The tuning with depth in the model is influenced by two competing factors, ?, the change in Coriolis parameter with latitude, and stretching. When stretching is dominant, the flow turns in the opposite direction. The model is extended to more than two layers with similar results. The model is compared with conditions found in the Kuroshio in wintertime. Because of the weak stratification and resultant short deformation radius, stretching is shown to be the dominant reaction to changes in potential vorticity caused by cooling. Thus, cooling causes water columns to stretch and the flow to turn to the left with depth. This result is consistent with subtropical mode water as a cooling-induced thickening of the surface mixed layer.